This invention relates generally to electrical logging of formations surrounding a borehole; more particularly, it relates to measuring formation resistivity by processing signals induced in receiving antennae by electromagnetic waves that are caused to propagate through the formation by transmitting antennae.
The prior art shows certain methods and apparatus for such logging of formation resistivity. Examples are known both for independent logging operations and for logging during measure while drilling (MWD) operations. Typical examples are shown by U.S. Pat. Nos. 3,891,916, 4,107,598 and 4,513,693. In these examples a number of antennae, some transmitting and some receiving, are disposed on the outside of, or embedded in the outer surfaces of, a drill collar or other elongated tubular member. Electronic equipment located interior to the drill collar or elongated tubular member provides power excitation to transmitting antennae, typically in the frequency range of a few hundred kilohertz to a few megahertz. Electromagnetic energy transmitted through the borehole formation exterior to the collar or tubular member is sensed by receiving antennae. Signals from such receiving antennae are processed by electronic equipment interior to the collar or tubular member to provide data that is indicative of the formation resistivity of the material surrounding the borehole. Generally, such signal processing measures the phase velocity (phase difference) and the attenuation (amplitude ratio) of the electromagnetic wave energy as it propagates past the receiving antennae.
The external placement of the antennae leads to various problems with mechanical damage to such antennae in use. Several outer shielding or covering approaches are known to provide some protection to the antenna structures. Recently, U.S. Pat. No. 5,530,358 describes and claims an improved antenna system for resistivity tools, in which antenna elements are embedded within the drill collar or elongated tubular member. The antenna elements are exposed to the exterior region by a plurality of discrete communication regions. These discrete communication regions are elements of a contoured portion of the outer peripheral surface of the collar or tubular member, and that have a reduced radial dimension. The antenna coils are disposed circularly around the elongated tubular member at a radius between that of the outer surface and the inside radius of the slots. The objective of this configuration is to provide protection to the antenna elements. Further, means are known to provide for directional variation of the antenna pattern.
U.S. Pat. No. 5,682,099 describes, but does not claim, a drill collar or measurement tubular for a resistivity tool having slots completely through the collar from the exterior to the interior in the region of the antennae but does not show or suggest that such a configuration permits a retrievable sonde or tool. U.S. Pat. No. 5,939,885 describes and claims a tubular body member having apertures therein and a mounting member comprising two portions to permit the antenna to be mounted or removed from the mounting member. Again, there is no indication that such a configuration can be used to provide a retrievable resistivity sonde or tool.
One significant problem all such prior art approaches is that the antenna elements are fixed, in one way or another, within or exterior to the collar or tubular element. Thus, they can only be removed from the borehole by completely withdrawing the entire drill string from the borehole. With this approach, it is impossible to withdraw an antenna structure to correct a failure or to remove the complete logging apparatus or to change the antenna structure to a different configuration without the added cost and time required for pulling the entire drill string from the borehole.
The shortcomings of existing technologies are well known and there is a need for improvements to such resistivity tools to overcome them. It is the objective of this invention to provide an improved resistivity tool that overcomes some of the difficulties and expense associated with the prior art tools.
This invention provides a resistivity measurement tool that is entirely contained within the interior of the drill collar or elongated tubular member. As such, the complete resistivity tool can be retrieved as a unit, antennae as well as electronic elements.
Careful electromagnetic and structural analyses have shown that suitable resistivity logging and suitable mechanical strength can be obtained in a structure similar to that shown at FIG. 3AA in U.S. Pat. No. 5,682,099 that has slots, of several possible configurations, extending completely through the collar or elongated tubular member from the exterior surface. Antenna elements located completely internal to the collar and in the region of the slots provide suitable coupling of energy into the formation, and detection of such signals resulting from such transmission through the formation, to obtain the desired measurement of formation resistivity.
In one embodiment, a number of slots are cut through the entire thickness of the collar or elongated tubular member symmetrically over each of the antenna elements. Such slots may, for example, be axial in direction and generally of much greater length along the borehole axis than the width of such slots. The effectiveness of the antenna elements interior to the collar depends on the number of slots, the length of the slots, the width of the slots and the axial location of the antenna elements with respect to the slotted region. The mechanical strength of the collar or elongated tubular member depends on the number of slots, the length of the slots and the width of the slots. Detailed finite element analysis of both the electromagnetic effectiveness and the mechanical strength for various combinations enables selection of those combinations that are suitable for the intended purpose of formation resistivity measurement wile maintaining adequate mechanical strength of the string.
The slots may be filled with epoxy, ceramic or other insulating material to provide a seal to prevent formation fluids from flowing into the internal apparatus. Slots may be tapered so as to be wider on the interior side than on the exterior side so that the normally higher internal pressure will force the slot filler into the slot. Although axial slots represent the preferred embodiment, other configurations are possible. Slots may be diagonal, crosswise or zigzag with respect to the borehole axis. Slots may generally be in the range of 0.05 to 0.5 inches in width and on the order of 4 inches long. The number of slots may range from a low of one to as many as twenty to thirty or more for each antenna region location.
The general electronic structure for this invention is known. Electronic components and functions include circuits for power conditioning, for generation of the desired high frequency power to be applied to the transmitting antennae, for the detection of the desired signals from the receiving antennae, for processing such received signals to determine measures of formation resistivity and for conditioning such formation resistivity data to a form suitable for transmission to the surface or for storing in other downhole apparatus. One or more frequencies in the range from about 100 KHZ to about 4 MHZ may be generated and transmitted into the formation. Data output from the formation resistivity tool may be transmitted to the surface by well known means such as mud pulse telemetry, electromagnetic telemetry or conventional wireline.
The use of a slotted collar or elongated tubular member permits provision of an integrated and separate antenna tool or sonde having all of the elements in a single unit which is one important objective of the invention. Such an integrated tool reduces cost of manufacture, while maintaining retrievability from the interior of the drill string, without pulling the entire drill string resulting in substantial cost savings in use. The integrated tool can be used inside different other structures, and since it is complete including its antennae, it can be used for formation resistivity logging in an open hole.
Accordingly, a major object is to provide apparatus for the measurement in a borehole of resistivity of the formation external to the borehole, comprising:
a) longitudinally elongated tubular structure adapted to be located in the borehole, and having exterior and interior surfaces and a selected configuration of a slot or slots extending completely through the structure between its exterior surface and its interior surface,
b) a sonde or tool completely contained within the interior of the tubular structure to be relatively movable lengthwise therein, and having:
1. one or more electromagnetic energy transmitting antennae positioned proximate a certain slot or certain of said slots,
2. one or more electromagnetic energy receiving antennae positioned proximate another slot or other selected slots,
3. and electronic elements to provide radio frequency power to the transmitting antennae to cause transmission of electromagnetic energy into the sub-surface formation via said certain slot or certain of the slots, receiving means to receive radio frequency signals arriving at the receiving antennae from the sub-surface formation via said other slot or other selected slots.
In addition, processing means are typically operatively connected to output from the receiving means to provide data indicative of formation resistivity at the exterior of the collar or tubular structure, and means is provided to communicate such data to other equipment, or to store the data.
Also provided is structure to support elements of the sonde or tool, and attachment means for moving the sonde with respect to the collar.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: